[Updating Arrays in State] examples in the beta documentation are using impure function to explain concepts which doesn't seems right

Hi, learning react and going through the new react doc (beta).
I finished reading about pure functions, but then in a later section (update arrays in state), it showcase multiple examples that seem contradictory to the concept of pure functions in React...
OR I simply did not really understand that concept and If that is the case, I am sorry in advance.

let nextId = 0;

export default function List() {
  const [name, setName] = useState('');
  const [artists, setArtists] = useState([]);

  return (
    <>
      <h1>Inspiring sculptors:</h1>
      <input
        value={name}
        onChange={e => setName(e.target.value)}
      />
      <button onClick={() => {
        setName('');
        artists.push({
          id: nextId++,
          name: name,
        });
      }}>Add</button>

  //rest of the code...

isnt using a variable from outside (in this case: let nextId = 0) and just modifying it against the concept of pure function in react?
If this is indeed an impure function, I just think it would be better to promote using pure functions in the visual examples just to not confuse new learners or at least mention that it was made on purpose.

Comece pela seção “Adding to an array” na documentação vinculada “Update Arrays in State” e compare os exemplos dela com as orientações sobre Pure Functions. Determine se os exemplos de nextId e artists.push precisam de esclarecimento ou substituição e considere o trabalho concluído quando a documentação não confundir mais esses conceitos.